Steel and cast iron are both in the -0.60 to -0.71 Volt range in the galvanic series so it appears that there should not be much of a corrosion potential with steel foil type head gaskets. I seem to recall
the old Yanmar
had a copper foil type gasket and copper is -0.30 to -0.57 Volt which is more of a potential difference and I did not have any problem with it.
The biggest problem I had was removing the alloy timing case cover from the raw water path through the engine. I used an interference
fit nylon bushing expanded in the cast iron block to do this but I don't know how long this will last before it starts leaking.
I was going to replace the original water pump housing when I convert to keel cooling but am now considering leaving the existing arrangement and fitting another small impeller pump onto the female threaded input shaft on the back of the gear
box to supply raw water for the exhaust system.
The alternative is go to a dry exhaust system which removes another source of possible sea water leakage in the steel boat and completely removes the exhaust manifold engine back flooding hazard.
If I've pressed the right keys the two images
show the front of the engine and the home made water cooled exhaust manifold.
I used cupro nickel tube and fittings and silver soldered all the piping because I don't like hoses running everywhere. I used t's instead of elbows and the plugs on the t's are anodes.
The water cooled manifold is manufactured from a piece of 2" x 2" steel SHS with SS sleeves for the manifold bolts and a piece of 3" x 2" SS RHS sleeved over the 2" x 2" black steel exhaust tube. The whole thing was manufactured with hand tools and a drill press on the boat.
I'll dig about and see if I can find some photos of the aborted manifold as they show the fabricated bell housing with the dual starter motor arrangement.
To answer the original enquiry, not only can you turn pretty well any engine into a marine engine, you can do it on the boat with a few hand and portable power tools. The result may not look as tidy as the shop version but there is a good chance it will work and the most enjoyable and satisfying boating
is that which includes lots of your own inputs.
Ah decisions, decisions, decisions.